This invention relates to a tire, particularly a truck tire, and particularly a bus tire, intended to be used to support and carry relatively large vehicular loads over roads, which may contain irregular road surfaces. The lugs of the tire tread may be in a form of circumferential rubs. Rib treaded tires have been referred to, for example, and not intended to be limiting, in U.S. Pat. Nos. 5,718,782, 5,772,807 and 5,843,249.
It is desired that the outer tread cap rubber layer of the tread, which contains the tread's running surface, is a rubber composition intended to promote durability of the tread, particularly for use on irregular road surfaces.
In one embodiment, the tread cap rubber composition is a natural rubber-rich rubber composition where a major portion of its elastomer is comprised of natural cis 1,4-polyisoprene rubber and a minor portion comprised of at least one of polybutadiene (e.g. c is 1,4-polybutadiene) and styrene/butadiene elastomers.
In another embodiment, the tread cap rubber composition is a polybutadiene rubber-rich (e.g. cis 1,4-polybutadiene-rich) rubber composition with a major portion of its elastomer is comprised of the polybutadiene rubber and a minor portion comprised of at least one of cis 1,4-polyisoprene and styrene/butadiene elastomers.
Where lower rolling resistance is desired for such truck, or bus, tire to promote vehicular fuel economy, the outer tread cap rubber layer may contain reinforcing filler composed of a combination of rubber reinforcing carbon black and precipitated silica where a significant portion of the reinforcing filler is precipitated silica. If the precipitated silica exceeds the rubber reinforcing carbon black in the outer tread rubber composition, the rubber composition might be referred to as being a silica-rich rubber.
During service of the tire, particularly over irregular road surfaces, the tread lugs, particularly circumferential tread ribs, may experience significant physical stress which may sometimes promote a degree of small surface crack formation on the tread groove surfaces between the tread lugs, or circumferential tread ribs, particularly in the bottom portion of the tread grooves.
Historically, surfaces of tread grooves have sometimes been protected, when appropriate and if desired, by providing an external thin protective rubber layer over the surface, or surfaces, of the grooves. Further, in one aspect, tread grooves have sometimes been reinforced with stiffer rubber than the tread itself to promote handling of the tire. For an example, which is not intended to be limiting, see U.S. Pat. Nos. 6,213,181 and 5,176,765 and U.S. Patent Application Publication No. 2010/0154948.
However, as a departure from such past practice, it is proposed to provide a rubber block, or encasement, within the tire tread for containing at least a portion of at least one of the tread grooves instead of being a simple rubber coating on the groove surface where a rubber block extends from the tire's carcass through the tread's base rubber layer into the tread's outer cap rubber layer to thereby encompass at least a portion of a tread groove. In this manner, then, for a circumferential groove, the rubber block itself would also be circumferential in a sense of extending circumferentially around the tire within the tire tread. The groove containing encasement, or rubber block, would thereby become at least a portion, or part, of the surface of the tread groove itself.
In one embodiment, the rubber encasement, namely the rubber block forming the encasement, joins and extends from the tire carcass axially outward through the tread base layer through the outer tread cap rubber layer to and including the outer running surface of the tire tread (tread cap layer). Therefore the encasement, or rubber block, is not simply a thin rubber coating over a surface of the tread groove, but instead is a part of the tread groove itself.
In another embodiment, the rubber encasement, namely the rubber block forming the encasement, extends from tire carcass axially outward through the tread base layer into the outer tread cap rubber layer without extending to the tread running surface.
As indicated, the groove containing rubber block (rubber encasement) is a part of at least a portion of a tread groove and desirably includes the bottom of the tread groove.
The rubber composition of the groove containing rubber block (encasement) beneficially promotes at least one physical property such as greater tear resistance, greater cut growth resistance and increased elongation at break to the surface of the tread groove embedded in the rubber encasement, or rubber block, compared to the adjoining outer tread cap rubber composition which contains the rubber block. The rubber composition of the tread groove containing rubber block may also beneficially promote increased fatigue resistance of the surface of the embedded tread groove as compared to the adjoining outer tread cap rubber composition.
In one embodiment, rubber composition of the tread cap rubber layer may contain silica-rich reinforcing filler comprised of a combination of rubber reinforcing carbon black and precipitated silica of which the precipitated silica is the majority of the rubber reinforcing filler. The precipitated silica is used together with silica coupling agent In one embodiment, the rubber reinforcing carbon black content of the tread cap rubber composition may be less than 30 parts by weight per 100 parts of elastomer (phr) which, in turn, can promote a significant reduction of its electrical conductivity (can promote an increase its electrical resistivity).
In one embodiment, the tread base rubber composition contains carbon black-rich filler reinforcement of which the rubber reinforcing carbon black is the majority of the rubber reinforcing filler. Therefore the rubber reinforcing carbon black is the majority of the reinforcing filler, usually in a quantity of at least 50 phr thereof. When the rubber reinforcing carbon black content in the rubber composition is at least 40 phr is expected to promote electrical conductivity (promote a reduction in electrical resistivity).
The rubber composition of the rubber encasement, or tread groove-containing rubber block extending from the tire carcass may be of the same or different rubber composition as the portion of tire carcass from which its extends and the same or different from the tread base rubber layer through which it passes.
When the tread groove containing rubber block contains at least about 40 phr of rubber reinforcing carbon black which extends from the tread carcass rubber which contains at least 40 phr of rubber reinforcing carbon black extends to an outer tread rubber cap which contains less than 35, alternately less than 20, phr of rubber reinforcing carbon black a path of least electrical resistance (path of electrical conductivity) can thereby be provided between the tread carcass and outer tread running surface of the tread cap rubber layer by the groove containing rubber block.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.